Energy
Electricity
Use Ohm's law and the series and parallel rules to find a resistance or a current, work out electric power and energy, and say why a heater's element is an alloy and a fuse is a weak link.
Flick a switch and the room lights up. What the switch does is complete a loop — a continuous closed path — and charge begins to flow round it. Break the loop anywhere and the flow stops everywhere, which is the first thing to know about a series circuit and the reason a single dead bulb can take out a whole string of fairy lights.
One law, three shapes
Ohm's law says the potential difference across a metallic wire is proportional to the current through it, so long as its temperature does not change. Written three ways: V = IR gives the voltage, R = V/I gives the resistance, and I = V/R gives the current. Resistors in series add up. Resistors in parallel add up as reciprocals, and the combination comes out easier to push current through than any one of them alone.
Worked example
Why is a heater's element made of an alloy, while the filament of a bulb is tungsten?
An alloy generally has a higher resistivity than the metals it is made of.
Higher resistance at the same voltage means more heat produced, which is the whole point of a heating element.
Try it together
Five resistors are wired up. A 10 Ω and a 40 Ω sit in parallel as one group; a 30 Ω, a 20 Ω and a 60 Ω sit in parallel as another; the two groups are joined in series.
Answer with a bare number each time, in ohms.
1.Take the first group. 1/R = 1/10 + 1/40. What is R for that group?
Have a go
Have a go on your own. A potential difference of 36 V is applied across a total resistance of 18 Ω. What is the current, in amperes? Answer with a number.
Ready to practice?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.